1. Field of the Invention
The present invention generally relates to a planar multiband antenna, in particular to an aperture-coupled circularly polarized planar dual-band antenna which can be employed in ISM bands from 2.40 GHz to 2.48 GHz and 5.15 GHz to 5.35 GHz.
2. Description of the Related Art
Wireless systems which have to function in several frequency bands are being developed more frequently. Frequently, compact antennas are necessary to keep the setup volume of the antennas small and to allow usage in portable devices.
It is possible to provide a separate antenna for each frequency band to be used. The disadvantage of using separate antennas, however, is that a multiplexer has to be employed. In addition, the area necessary for the antennas increases when using separate antennas.
Receiving from several different wireless transfer systems by a single broadband antenna is problematic since broadband antennas cannot usually be manufactured at low cost in a compact design. If all the relevant systems are to be received by a single broadband antenna, this will not be possible using a small cheap antenna.
A multi-element antenna having a special radiator for every frequency range can be used for receiving several frequency bands. Most antenna concepts known which are suitable for receiving from two or more frequency bands (dual-band concept and/or multiband concept) and which can be used for and/or in patch antennas, such as, for example, integrated inverted-F antennas (IFAs) and planar inverted-F antennas (PIFAs), comprise only a linear polarization. Well-known antenna shapes of this kind are, for example, described in the book “Planar Antennas for Wireless Communications” by Kin-Lu Wong (John Wiley & Sons, Inc., Hoboken, N.J., 2003).
However, it is desirable in particular for mobile applications to use a circular polarization, since in this case the orientation of transmitting and receiving antennas is uncritical, whereas when using linear polarization, the orientation of the antennas has to be selected appropriately.
A series of antennas which may be integrated comprising a circular polarization are known, however many of the geometries which may be integrated comprise essential disadvantages for generating a circular polarization. Exemplarily, nearly squared patches (planar conductive areas) of coaxial feeding have a low impedance bandwidth, as is, for example, described in the dissertation “Untersuchung und Aufbau von Multibandigen Antennen zum Empfang zirkular polarisierter Signale” by U. Wiesman produced in 2002 at the Fraunhofer-Institut für integrierte Schaltungen in Erlangen. Another multiband antenna of coaxial feeding is described in the article “A Dual Band Antenna for WLAN Applications by Double Rectangular Patch with 4 Bridges” by Chang Won Jung and Franco De Flaviis, published by the Department of Electrical Engineering and Computer Science, University of California, Irvine, Irvine, Calif., 92697, USA.
One way of setting up a circularly polarized dual-band antenna is using aperture coupling. Such a solution is described in the article “A Dual-Band Circularly Polarized Aperture-Coupled Stacked Microstrip Antenna for Global Positioning Satellite” by D. M. Pozar and S. M. Duffy, published in IEEE Transactions on Antennas and Propagation, Vol. 45, No. 11, in November 1997. However, it is employed for broadband antennas of a resonant frequency or for antennas of several resonant frequencies close to one another, but not suitable for being employed in connection with multiband antennas.
The European patent document EP 1 072 065 B1 shows a double-band antenna for GSM and DCS having double polarization. Antenna elements stacked above one another are fed by a cross-shaped aperture in the reflector device. Microwave energy is guided by a coupling area segment and an also cross-shaped aperture in a first radiating area segment to a second radiating area segment. The disadvantage of such an antenna assembly is that in this antenna two feeding channels have to be combined by a quadrature hybrid broadband extension line coupler in order to generate a circular polarization. In addition, the European patent document does not mention purity of polarization and impedance bandwidth.
The European patent application EP 1 353 405 A1 suggests an antenna for two frequency bands (dual-band antenna) which is suitable for both the GSM 900 band and the GSM 1800 band and the UMTS band and is based on a single radiator type. The individual antennas comprise a metallic box open to the top and feeding through conductive traces and/or conductor patterns. The individual radiators are further implemented to comprise an octahedral aperture in the center and can consequently be placed one above the other. The disadvantage of the antennas described is that they comprise a complicated and not completely planar structure.
In summary, it can be stated that no antenna design which is easy as far as the technology involved is concerned and can be realized at low cost is known which allows, with good efficiency and sufficient bandwidth, radiation of a circularly polarized electromagnetic wave in two different frequency bands.